29th International Cosmic Ray Conference Pune (2005) 00, 101–104

The Telescope Array Low Energy Extension (1):the 6-km Stereo Detector
C. C. H. Jui

for the TA/TALE Collaboration(a) Dept. of Physics, University of Utah, Salt Lake City, UT 84112, U.S.A.

Presenter: C. C. H. Jui (jui@physics.utah.edu), usa-jui-CCH-abs1-he15-oral
We plan to build a series of detectors to extend the energy coverage of the Telescope Array (TA) experiment
to lower energies, called the TA Low energy extension (TALE). We plan to cover the energy range     eV with this experiment. Here we present the design of a double-stereo detector to be constructedby redeploying 50 of the existing mirror units from the High Resolution Fly’s Eye (HiRes) Experiment inconjunction with two of the fluorescence detector (FD) stations of the TA experiment. This arrangement will
extend the threshold for stereoscopic fluorescence measurements of ultrahigh energy cosmic ray (UHECR)
showers down to   eV.

1.

Introduction

The Telescope Array (TA) Experiment, now under construction in Millard County, Utah, U.S.A., is a hybridarray consisting of three fluorescence detector (FD) stations and 576 scintillation surface detector (SD) units.
The SD units are arranged in a square grid with 1.2 km spacing and the FD stations are located at the cornersof the surface array. This arrangement of the surface array gives TA an effective physics threshold of   eV.
Its fiducial area of 800 km  above this threshold is fully-efficient, and is completely covered by the threefluorescence stations in hybrid mode. The standard TA detectors are optimized for the measurement of theultrahigh energy cosmic ray (UHECR) spectrum in the region of the GZK suppression [1].
The TA Low energy extension (TALE) plans to make hybrid and stereo fluorescence measurements of cosmic
rays down to an energy of    eV, using additional fluorescence stations from the redeployment of existing
detector units from the High Resolution Fly’s Eye (HiRes), plus additional equipment to be constructed. TheHiRes experiment has been operating since spring, 1997 with one detector site (monocular mode), and since
the end of 1999 with two detector sites (stereo mode). HiRes is scheduled to suspend operation in Dugway,
Utah during the summer season of 2006. We plan then to move the detector units to the TA site. With 50 of theHiRes mirror units spread over two sites located 6km from standard TA FD stations, we can achieve total stereo
aperture about an order of magnitude larger than that of HiRes at   eV, while maintaining an aperture equal
to that of HiRes at   eV. This will allow us to make significantly better stereo measurements of the ankle
or dip region than was previously possible. The significance of the physics of the ankle region is discussed
elsewhere[2], as is the details of the tower detector for extending the observations down to     eV[3]. The
layout and performance of the 6km TALE detectors will be described in the following sections.

2.

Motivation

Both the Fly’s Eye and HiRes experiments have observed the ankle structure at ! eV[4, 5, 6]. It should
be noted, however, that the ankle was resolved only in the stereo energy spectrum of Fly’s Eye, whereas theFly’s Eye monocular measurement did not have sufficiently fine energy resolution to reveal this feature. The
monocular spectrum of HiRes, as shown in figure 1, did confirm the occurrence and location of the ankle feature
seen in the Fly’s Eye stereo spectrum. The HiRes experiment,however, was designed for stereo observations
above "$#%   eV. From  to &  eV, the stereo aperture of HiRes drops by more than two orders of102

C. C. H. Jui

10 km
13 km

Figure 1. HiRes monocular spectra[6] overlaid with the Fly’s Eyestereo spectrum[4] showing the ankle/dip structure near ')(+*-,/. 0 eV.
As is usually done for UHECR spectra, the differential flux has been
multiplied by 12 to reveal the finer structures.

Figure 2. Illustration of the limited stereo aperture
for the HiRes detector at '( *-, eV.

magnitude. This precipitous drop will make it difficult to make systematically reliable aperture calculation andstereo flux measurement in the ankle region.
Figure 2 illustrates this limitation of HiRes at   eV. To first order approximation, the collection area of afluorescence station can be modeled as a circle whose radius is determined by the largest distance at which it
can detect a vertical cosmic ray shower of a particular energy. For the HiRes detectors, the effective radius at
&  eV is 10 km. The two HiRes detectors are located at a distance of 13 km apart. The overlap (stereo) region
between the two circles is limited and shrinks much more quickly in area than either circle with decreasingenergy.

3.

Simulation Study of the TALE 6 km Configuration

The standard TA mirrors are about 50% larger than the HiRes mirrors and have an effective radius of 13 kmat &  eV. However, each TA FD only covers 108 degrees in azimuth, as required for hybrid operations. A
number of different locations are available at between 4-6 km from the two TA FD sites to the south of the array.
A simulation study was made of HiRes mirrors placed at 6 km in the center of the field of view of a TA FD site.
Not surprisingly, we found that most of the stereo aperture at   eV occurs in those HiRes mirrors facing
away from the TA sites. However, those mirrors facing directly away from TA sites capture mostly events withsmall opening angles between the two shower-detector planes and result in poor geometrical reconstruction.
Removing these, we arrive at a two-site configuration shown in figure 3.
The intended optimization of the stereo aperture at  eV is evident in the left panel of figure 4, in which wehave represented the collection areas of the five TA/TALE FD sites by wedges. In the right panel of figure 4,
we also see the overlap at   eV, which demonstrates that the TALE 6 km upgrades will also provide TA103TALE (1): 6km stereo Detector

To TA FDTo TA FD

Figure 3. Arrangement of HiRes mirrors at 6 km, in the center of view of the TA FD sites. The individual mirrors areseen in top-projection view, covering elevation angles from 35463' 7 . The site on the right also shows the 15 tower mirrors
placed to view higher elevations

with stereo-hybrid coverage over essentially the entire surface array at above   eV. The expected angularresolution for stereo-hybrid events is about 8:9 . The combination of the large collection area and the extrafine angular resolution will make TA/TALE a uniquely powerful tool for searching for point-sources of UHE
cosmic ray.

Figure 4. Illustration of the overlap of the collection areas between the TALE mirrors (located at squares) and the TA FD
stations at ')( *-, eV (left) and at ')( *-; eV (right). The blue octagon in both panels represents the extent of the TA surface
array, and the outer magenta pentagon shows the proposed Northern AUGER site in Utah.

The simulations also show the stereo angular and <>=@?)A resolutions of TA/TALE to be equivalent to that ofHiRes. The combined stereo aperture, plotted in figure 5, is roughly equal to that of HiRes at    eV, anddecreases six-fold to  eV. This represents an improvement of about an order of magnitude over HiRes atthe low end of the ankle region.

104

C. C. H. Jui

Figure 5. Total calculated stereo aperture of the TALE 6 km detectors between ')( *CB4.

4D'( *-;eV.

Summary

With the modest investment of refurbishing and moving existing HiRes detectors from Dugway to MillardCounty, TA/TALE will be able to extend stereo fluorescence observations for TA down to   eV. By locating
the stereo pairs 6 km apart, the resulting aperture over the ankle region will increase by about a factor of six
between EF&  eV, which is significantly flatter than that of HiRes. In addition, this arrangement will
also result in the world’s largest stereo-hybrid detector with a collection area of 800 km  at above  eV andan anticipated angular resolution of about  9 .

5.

Acknowledgments

We thank colleagues from the AGASA, HiRes, and TA collaborations for valuable discussions. The Telescope
Array is being constructed by the support of Grant-in-Aid for Scientific Research (Kakenhi) on the Priority
Area ”The Highest Energy Cosmic Rays” by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We also thank the University of Utah, the State of Utah, and the Utah Economic Development
Board for their generous support.

References[1] K. Griesen Phys. Rev. Lett. 16, 748 (1966).
G.T. Zatsepin and V.A. Kuzmin, JETP. Lett. 4, 78 (1966).
[2] G B. Thomson (TA/TALE Collaboration), to appear in the Proc. of the 29th ICRC, Pune, (2005).
[3] G B. Thomson (TA/TALE Collaboration), to appear in the Proc. of the 29th ICRC, Pune, (2005).
[4] D.J. Bird et al., Phys. Rev. Lett. 71, 3401 (1993).
[5] R. U. Abassi et al. (HiRes Collaboration) Phys. Rev. Lett. 92 (2004) 151101 (arXiv:astro-ph/0208243)[6] R. U. Abassi et al. (HiRes Collaboration) Phys. Letters B, in press (2005) (arXiv:astro-ph/0501317)